Orthogonality detector, and quadrature demodulator and sampling quadrature demodulator using detector thereof

ABSTRACT

To detect phase mismatches between in-phase and quadrature signals of a quadrature demodulator. The phase mismatches can be detected using the signals obtained by removing high frequency components of output of a multiplier by a low pass filter, the output being the product of the in-phase signals of which low frequency components are removed by a first high pass filter by the quadrature signals of which low frequency components are removed by a second high pass filter.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2004-264802 filed on Sep. 13, 2004, the content of which is hereby intothis application.

FIELD OF THE INVENTION

This invention relates to a quadrature demodulator in digitalcommunication, in particular to a quadrature demodulator performingphase mismatch compensation using an orthogonality detector and asampling quadrature demodulator performing group delay differencecompensation.

BACKGROUND OF THE INVENTION

In the past, as a method for compensating conversion gain error, afeedback loop was used as a structure for calculating difference of theamplitudes of in-phase signals and quadrature signals and amplifying theamplitude of the in-phase or quadrature signals so as to make thecalculation result to be zero (for example, Patent document 1). Also inthe past, as receiver architecture, sub-sampling in an A/D(analogue/digital) converter was used (for example, Patent document 2).Furthermore, in the past, as a method for compensating phase mismatch,orthogonality was detected by calculating the product of in-phasesignals (I) by quadrature signals (Q) and finding out their crosscorrelation (for example, Non-patent document 1).

[Patent document 1] Japanese Unexamined Patent Application PublicationNo. 2003-258931

[Patent document 2] Japanese Unexamined Patent Application PublicationNo. 2003-318759

[Non-patent document] Behzad Razavi, ‘design Considerations forDirect-Conversion Receivers’, IEEE Transaction on Circuits and SystemsII, vol. 44, no. 6, pp. 428-435, 1997

SUMMARY OF THE INVENTION

FIG. 11 shows a block diagram of a receiver structure in directconversion manner. From the signals received by an antenna 251,frequency components other than frequency bands to be received arefiltered out by a band pass filter 261 for band selection, and thesignals are input into an LNA (low noise amplifier) 271 for improvingnoise figure of the entire receiver. The signals of outputs of the LNA271 other than desired frequency bands are removed by a band pass filter263 for channel selection, and the outputs are amplified by an AGC(Automatic Gain Control) amplifier 281. The outputs of the AGC amplifier281 are converted into in-phase signals and quadrature signals at aquadrature demodulation section consisting of a local oscillator 151, afixed phase shifter 171, mixers 142, 143, and low pass filters 132, 133.The frequency components of outputs of the mixers 142, 143 other thandesired bands are removed by low pass filters 132, 133 for channelselection. The outputs of the low pass filters 132, 133 are synchronizedwith the outputs of the local oscillator 151 and converted into digitalsignals by A/D (analogue/digital) converters 201, 202. Then the outputsof the A/D converters 201, 202 are processed by a digital signalprocessor.

On the other hand, as receiver architecture, there used to be a mannerdifferent from the direct conversion manner as shown in Patent document2. In this manner, sub-sampling in an A/D (analogue/digital) converteris used for quadrature demodulation. Hereinafter, this receiverarchitecture is called sampling quadrature demodulation manner.

FIG. 12 shows a block diagram illustrating the sampling quadraturedemodulation manner. From the signals received by the antenna 251,frequency components other than frequency bands to be received arefiltered out by the band pass filter 261 for band selection, and thesignals are input into the LNA 271 for improving noise figure of theentire receiver. The frequency components of the outputs of the LNA 271other than predetermined bands are removed by a band pass filter 262,and the carrier frequency of the outputs is converted by inputting theoutputs into the mixer 141 together with the outputs of the localoscillator 152. The signals of outputs of the mixer 141 other thandesired frequency bands are removed by the band pass filter 263 forchannel selection, and the outputs are amplified by the AGC amplifier281. The outputs of the AGC amplifier 281 are converted into in-phasedigital signals and quadrature digital signals at a quadraturedemodulation section consisting of the local oscillator 151, the fixedphase shifter 171, and A/D converters 201, 202. Since the outputs of theA/D converters 201, 202 are shifted by ¼ sample in sampling timing, thetiming is synchronized by a latch 211 for enabling afterward digitalprocessing. Then the outputs of the latch 211 are processed by thedigital signal processor.

In quadrature demodulation in direct conversion manner or samplingquadrature demodulation manner, theoretically, conversion gains from RFsignals into in-phase signals and quadrature signals are identical andthe phase would be orthogonal. However, due to dispersion in performanceof a device, there would be errors in the conversion gains ororthogonality. Because of such errors, there was a problem of increasein the bit error rate at the time of receiving.

Taking receiving of QPSK modulation signals as example, this problem isdescribed. FIG. 13A shows constellation of received signals when thereare errors in the conversion gain. In a theoretical reception, symbolsare arranged on a circle, however when there are errors in theconversion gain, the symbols are arranged on an ellipse and in thiscase, there are errors in the amplitude between in-phase signals andquadrature signals. FIG. 13B shows constellation of received signalswhen there are phase mismatch. Since the symbols are arranged on anellipse as well when there are phase mismatch, in this case, there areerrors in the amplitude between in-phase signals and quadrature signals.There was a problem that such errors increase the bit error rate.

Patent document 1 discloses a method for compensating the conversiongain errors. In this method, a feedback loop is used as a structure forcalculating difference of the amplitudes of in-phase signals andquadrature signals and amplifying the amplitude of the in-phase orquadrature signals so as to make the calculation result to be zero.Non-patent document 1 discloses a method for compensating the phasemismatch. In this manner, the orthogonality is detected by calculatingthe product of in-phase signals (I) by quadrature signals (Q) using acircuit as shown in FIG. 9 and finding out their crosscorrelation=E[I·Q] (where, E[ ] indicates the average]. In FIG. 9, anorthogonality detector 101, a multiplier 121 and a low pass filter 131are shown. Compensation is performed by structuring a feedback forcontrolling a phase shifter of a quadrature demodulator so as to makethe cross correlation to be zero.

The conventional manner as shown in Non-patent document 1 had theproblem of long convergence time at detecting orthogonality of phasesbetween in-phase and quadrature signals. This is because base bandsignals include direct current components and low frequency components.Since detecting phase takes time in inverse proportion to the frequency,it would have an essential problem.

Sub-sampling quadrature demodulation manner includes latch operation,which is particular in this manner. Because of the latch, group delaydifference is created between in-phase signals and orthogonal signals(ideally ¼ sample time). This difference might cause increase in the biterror rate. However, so far, compensation of such group delay differencehas not been considered. Also since the orthogonality between in-phasesignals and quadrature signals is achieved by shifting the samplingtiming, phase mismatch and sampling timing errors would occurcoincidently. Accordingly, variable group delay difference compensationhas been required according to the phase mismatch.

To solve the above mentioned problems, according to the presentinvention, the convergence time is reduced by detecting theorthogonality of phase between in-phase and quadrature signals using thecovariance of the in-phase and quadrature signals. To be more concrete,a typical example according to the present invention is described below.An orthogonality detector according to the present invention comprises afirst high pass filter for removing low frequency components of a firstinput signal, a second high pass filter for removing low frequencycomponents of a second input signal, a multiplier for multiplying therespective outputs of the first and second high pass filters each other,and a low pass filter for removing high frequency components from theoutput of the multiplier, wherein the orthogonality between the firstand second signals is detected based on the outputs of the low passfilter.

According to the present invention, since the product is calculatedafter removing low frequency components, the orthogonality can bedetected quickly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an orthogonality detector of a firstembodiment according to the present invention;

FIG. 2 shows a block diagram of an example of a quadrature demodulatorhaving functions for compensating orthogonality using the orthogonalitydetector according to the present invention as a second embodiment ofthe present invention;

FIG. 3 shows a block diagram of another example of a quadraturedemodulator having functions for compensating orthogonality using theorthogonality detector according to the present invention as a thirdembodiment of the present invention;

FIG. 4 shows a block diagram of a further other example of a quadraturedemodulator having functions for compensating orthogonality using theorthogonality detector according to the present invention as a fourthembodiment of the present invention;

FIG. 5 shows a block diagram illustrating a quadrature demodulator insampling quadrature demodulation manner using group delay differencecompensation according to the present invention as a fifth embodiment ofthe present invention;

FIG. 6 shows a block diagram illustrating a quadrature demodulator insampling quadrature demodulation manner adopting group delay differencecompensation according to the present invention and phase mismatchcompensation using the orthogonality detector according to the presentinvention as a sixth embodiment of the present invention;

FIG. 7 shows a block diagram illustrating a quadrature demodulator insampling quadrature demodulation manner adopting variable group delaydifference compensation according to the present invention and phasemismatch compensation using the orthogonality detector according to thepresent invention as a seventh embodiment of the present invention;

FIG. 8 shows a block diagram illustrating a quadrature demodulator insampling quadrature demodulation manner adopting group delay differencecompensation according to the present invention, phase mismatchcompensation using the orthogonality detector according to the presentinvention, and compensation of conversion gain error as an eightembodiment of the present invention;

FIG. 9 shows a block diagram illustrating a conventional orthogonalitydetector;

FIG. 10 shows a block diagram in a case that the orthogonality detectoras in FIG. 8 is replaced to a conventional orthogonality detector;

FIG. 11 shows a block diagram of a receiver structure in directconversion manner;

FIG. 12 shows a block diagram of a receiver structure in samplingquadrature demodulation manner;

FIGS. 13A, 13B show diagrams illustrating constellation change in a casethere is a conversion gain error or a phase mismatch in quadraturedemodulation;

FIG. 14 shows a diagram illustrating convergence of phase error when thephase mismatches are compensated in respective structures as in FIGS. 8and 10;

FIG. 15 shows a diagram illustrating relation between cutoff frequencyof a high pass filter at the time of phase mismatch compensation and RMSphase error after convergence; and

FIG. 16 shows a diagram illustrating convergence of ratio of in-phasesignals and quadrature signals when the conversion gain is compensatedin the structure as in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An orthogonality detector according to the present invention isstructured so as to calculate covariance of two input signals and todetect orthogonality between the two signals from the calculatedcovariance. The covariance can be obtained by inputting the two signalsrespectively into a high pass filter to remove low frequency componentsfrom the two signals and by multiplying the respective outputs of thehigh pass filter each other by a multiplier. To be more specific, thecovariance can be obtained by further inputting the outputs of themultiplier into the low pass filter and by removing high frequencycomponents from the outputs of the multiplier.

The orthogonality detector according to the present invention detectsphase mismatches or amplitude errors between the detected orthogonalityby the detector itself. In the orthogonality detector for detectingorthogonality of in-phase and quadrature signals, the covariance betweenthe in-phase signals and quadrature signals is calculated by removinglow frequency components of the in-phase signals using a first high passfilter, removing low frequency components of the quadrature signalsusing a second high pass filter, multiplying the outputs of the firstand second high pass filters using a multiplier, and removing highfrequency components of the output of the multiplier using a low passfilter.

For group delay difference (ideally, ¼ sampling time) in sub-samplingquadrature demodulation manner, the in-phase and quadrature signals arecompensated by delaying the signals by a delay device or a digitalfilter having group delay functions and by providing delay difference tothe in-phase signals and quadrature signals. The group delay differencedepending on phase mismatches is compensated by delaying either or bothof the in-phase and quadrature signals by a variable delay device. Thedelay quantity to be compensated is determined according to the outputof the orthogonality detector based on the covariance. Themultiplication by the multiplier is statistically equivalent tocalculation of the covariance between a variable I and a variable Q.Covariance=E[(I−E[I])(Q−E[Q])]It is well known that when I and Q are uncorrelated, the covariancewould be zero. Also, when I and Q have close correlation, the covariancewould be larger. Applicants found out the property of the covariancethat based on the above mentioned principle, where the in-phasecomponents (I-components) of the signals are variables I and thequadrature components (Q-components) of the signals are variables Q, thestate of the in-phase components and quadrature components beingcompletely orthogonal is considered to be uncorrelated and thecovariance would be zero while the covariance would be larger as theorthogonality is lower. They further found out that the covariance canbe also used as an index for the orthogonality similarly as crosscorrelation and finally, they have proposed a concrete circuit structurefor detecting the orthogonality using above mentioned property as anorthogonality detector according to the present invention.

In detecting the orthogonality of in-phase and quadrature signals, thelow frequency components also contributed to phase detection since thedetection was performed based on a simple cross correlation andaccordingly, it took long time for phase detection. However, withrespective embodiments according to the present invention as describedbelow, since the product is calculated after removing the low frequencycomponents, the orthogonality can be detected quicker.

First Embodiment

FIG. 1 shows a block diagram of an orthogonality detector of a firstembodiment according to the present invention. The orthogonalitydetector 101, high pass filters 111, 112, a multiplier 121 and a lowpass filter 131 are provided. This orthogonality detector is differentfrom a conventional orthogonality detector based on the crosscorrelation as shown in FIG. 9 in the point of the high pass filters111, 112 being arranged thereon.

An orthogonality detector 101 comprises a first high pass filter 111 forremoving low frequency components of a first input signal, a second highpass filter 112 for removing low frequency components of a second inputsignal, a multiplier 121 for multiplying the respective outputs of thefirst and second high pass filters 101, 102 each other, and a low passfilter 131 for removing high frequency components from the output of themultiplier 121. The orthogonality between the first and second signalsis detected based on the outputs of the low pass filter 131.

Low frequency components of the two signals input into the orthogonalitydetector 101 are removed by the high pass filter, and the signals aremultiplied by the multiplier 121. Thus, the covariance can be obtained.High frequency components of the covariance obtained from the outputs ofthe multiplier 121 are removed by the low pass filter 131. The outputsof the multiplier 121 include high frequency components not required asindex of the orthogonality and occurred by signal modulation in additionto the covariance. For removing the high frequency components, the lowpass filter 131 is required.

Thus, the phase mismatches or amplitude errors between the first andsecond signals are detected from the orthogonality between the first andsecond signals detected in such a manner. According to the presentembodiment, orthogonality can be detected quickly compared to aconventional case based on the cross correlation by using the outputs ofthe low pass filter 131 as index of the orthogonality.

Second Embodiment

FIG. 2, illustrating a second embodiment according to the presentinvention, shows a block diagram of a quadrature demodulator having anphase mismatch compensation circuit for compensating errors betweenin-phase and quadrature signals using the orthogonality detectoraccording to the present invention. Components 101, 111, 112, 121, 131constitute an orthogonality detector as similarly as in the firstembodiment. Low pass filters 132, 133, mixers 141, 142, a localoscillator 151 and a variable phase shifter 161 constitute a quadraturedemodulation circuit section. A quadrature demodulator in the presentembodiment comprises the quadrature demodulation circuit section and theorthogonality detector into which the outputs of the quadraturedemodulation circuit section is input. It further comprises a circuitfor compensating phase mismatches between in-phase and quadraturecomponents of the outputs of the quadrature demodulation circuit sectionbased on the outputs of the orthogonality detector 101.

The orthogonality detector 101 comprises a first high pass filter 111for removing low frequency components of in-phase components of thesignals input from quadrature demodulation circuit section, a secondhigh pass filter 112 for removing low frequency components of quadraturecomponents of the signals input from the quadrature demodulation circuitsection, a multiplier 121 for multiplying the respective outputs of thefirst and second high pass filters 111, 112 each other, and a low passfilter 131 for removing high frequency components from the outputs ofthe multiplier 121, wherein the orthogonality between the in-phase andquadrature components of the signals input from the quadraturedemodulation circuit section is detected based on the outputs of the lowpass filter 131. The outputs of the quadrature demodulation circuitsection of which phase mismatches are compensated based on theorthogonality of the two signals detected in such a manner areconsidered to be the outputs of the quadrature demodulator.

The quadrature demodulation circuit section comprises the localoscillator 151, the first mixer 141 outputting in-phase components ofthe input signal of the quadrature demodulation circuit section, thesecond mixer 142 outputting quadrature components of the input signalsof the quadrature demodulation circuit section, and the variable phaseshifter 161 connected between the output terminal of the orthogonalitydetector 101 and the input terminal of either the first or second mixer141, 142. The variable phase shifter 161 changes phase shift quantity ofthe variable phase shifter 161 based on the outputs of the orthogonalitydetector 101 and controls input phase of any of the first or secondmixer 141, 142 to which the output terminal of the variable phaseshifter 161 is connected.

The input terminal of the variable phase shifter 161 of this embodimentis connected to the output terminal of the local oscillator 151 commonlywith any of the first or second mixer 141, 142, the output terminal ofthe variable phase shifter 161 is connected to the other one of themixers 141, 142, and the outputs of the local oscillator 151 of whichphase is controlled by the variable phase shifter 161 is supplied towhich of the first or second mixer 141, 142 the output terminal of thevariable phase shifter 161 is connected. FIG. 2 shows an example ofstructure of which the output terminal of the local oscillator 151 isconnected to the input terminal of the first mixer 141, and the outputterminal of a variable phase comparator 161 is connected to the inputterminal of the mixer 142. This embodiment is not limited only to thisstructure. It also includes, for example a structure of which the outputterminal of the local oscillator 151 and the output terminal of thevariable comparator 161 are exchanged each other, that is, the outputterminal of the variable comparator 161 is connected to the inputterminal of the first mixer 141, and the output terminal of the localoscillator 151 is connected to the input terminal of the second mixer142.

The orthogonality between the in-phase and quadrature signalsdemodulated by quadrature demodulator and output from the low passfilters is detected by the orthogonality detector according to thepresent invention. Orthogonality between the in-phase and quadraturesignals is compensated by controlling phase shift quantity of thevariable phase shifter 161 using the detected signals and adjusting thephase of oscillation signals input into the mixer 142. According to thepresent embodiment, orthogonality can be detected quickly compared to aconventional case based on the cross correlation by using the outputs ofthe low pass filter 131 as index of the orthogonality, and accordingly,a quadrature demodulator capable of quickly outputting signalsconsisting of the in-phase and quadrature components compensated in thephase mismatches can be realized.

Third Embodiment

FIG. 3, illustrating a third embodiment according to the presentinvention, shows a block diagram of another example of a quadraturedemodulator having an phase mismatch compensation circuit forcompensating errors of orthogonal phase between in-phase and quadraturesignals using the orthogonality detector according to the presentinvention. Components 101, 111, 112, 121, 131 constitute anorthogonality detector as similarly as in the first embodiment. Low passfilters 132, 133, mixers 141, 142, a local oscillator 151 and a variablephase shifter 161 constitute a quadrature demodulation circuit section.A quadrature demodulator in the present embodiment, as similarly as inthe second embodiment, comprises the quadrature demodulation circuitsection and the orthogonality detector into which the outputs of thequadrature demodulation circuit section is input. It further comprises acircuit for compensating phase mismatches between in-phase andquadrature components of the outputs of the quadrature demodulationcircuit section based on the outputs of the orthogonality detector 101.

The orthogonality detector 101 comprises a first high pass filter 111for removing low frequency components of in-phase components of thesignals input from quadrature demodulation circuit section, a secondhigh pass filter 112 for removing low frequency components of quadraturecomponents of the signals input from the quadrature demodulation circuitsection, a multiplier 121 for multiplying the respective outputs of thefirst and second high pass filters 111, 112 each other, and a low passfilter 131 for removing high frequency components from the outputs ofthe multiplier 121, wherein the orthogonality between the in-phase andquadrature components of the signals input from the quadraturedemodulation circuit section is detected based on the outputs of the lowpass filter 131. The outputs of the quadrature demodulation circuitsection of which phase mismatches are compensated based on theorthogonality of the two signals detected in such a manner areconsidered to be the outputs of the quadrature demodulator.

The quadrature demodulation circuit section comprises the localoscillator 151, the first mixer 141 outputting in-phase components ofthe input signal of the quadrature demodulation circuit section, thesecond mixer 142 outputting quadrature components of the input signalsof the quadrature demodulation circuit section, and the variable phaseshifter 161 connected between the output terminal of the orthogonalitydetector 101 and the input terminal of either the first or second mixer141, 142. The variable phase shifter 161 changes phase shift quantity ofthe variable phase shifter 161 based on the outputs of the orthogonalitydetector 101 and controls input phase of any of the first or secondmixer 141, 142 to which the output terminal of the variable phaseshifter 161 is connected.

The input terminal of the variable phase shifter 161 is connected to theinput terminal of the quadrature demodulation circuit section, theoutput of the variable phase shifter 161 is connected to the inputterminal of any of the first or second mixer 141, 142, and the inputsignal of the quadrature demodulation circuit section of which phase iscontrolled by the variable phase shifter 161 is supplied to which of thefirst or second mixer 141, 142 the output terminal of the variable phaseshifter 161 is connected. FIG. 3 shows an example of structure of whichthe input terminal of the quadrature demodulation circuit section isconnected to the input terminal of the first mixer 141, and the outputterminal of a variable phase comparator 161 is connected to the inputterminal of the mixer 142. This embodiment is not limited only to thisstructure. It also includes, for example a structure of which the inputterminal of the quadrature demodulation section and the output terminalof the variable comparator 161 are exchanged each other, that is, theoutput terminal of the variable comparator 161 is connected to the inputterminal of the first mixer 141, and the input terminal of thequadrature demodulation circuit section is connected to the inputterminal of the second mixer 142.

The orthogonality between the in-phase and quadrature signalsdemodulated by quadrature demodulator and output from the low passfilters is detected by the orthogonality detector according to thepresent invention. Orthogonality between the in-phase and quadraturesignals is compensated by controlling phase shift quantity of thevariable phase shifter 161 using the detected signals and adjusting thephase of reception signals input into the mixer 142. According to thepresent embodiment, orthogonality can be detected quickly compared to aconventional case based on the cross correlation by using the outputs ofthe low pass filter 131 as index of the orthogonality, and accordingly,as similarly as in the second embodiment, a quadrature demodulatorcapable of quickly outputting signals consisting of the in-phase andquadrature components compensated in the phase mismatches can berealized.

Fourth Embodiment

FIG. 4, illustrating a fourth embodiment according to the presentinvention, further shows a block diagram of another example of aquadrature demodulator having an phase mismatch compensation circuit forcompensating errors of orthogonal phase between in-phase and quadraturesignals using the orthogonality detector according to the presentinvention. Components 101, 111, 112, 121, 131 constitute anorthogonality detector as similarly as in the first embodiment. Low passfilters 132, 133, mixers 141, 142, a local oscillator 151 and a fixedphase shifter 171 constitute a quadrature demodulation circuit section.A multiplier 122 and an adder 191 constitute a phase compensator 181.

The orthogonality between the in-phase and quadrature signalsdemodulated by quadrature demodulator and output from the low passfilters is detected by the orthogonality detector according to thepresent invention. Orthogonality between in-phase and quadrature signalsis compensated by controlling the phase compensator 181 using thedetected signals. The following relation as shown by [Formula 1] can beprovided between in-phase signal (I) and quadrature signal (Q) which canbe obtained when the phase mismatches of the quadrature demodulator areat the angle of θ and in-phase signal (I_(ideal)) and quadrature signal(Q_(ideal)) which can be obtained in an ideal quadrature demodulation.

$\begin{pmatrix}I_{ideal} \\Q_{ideal}\end{pmatrix} = {\begin{pmatrix}1 & 0 \\{\tan\;\theta} & {\sec\;\theta}\end{pmatrix}\begin{pmatrix}I \\Q\end{pmatrix}}$When the phase mismatches are small enough, the above formula would beequivalent to [Formula 2] since tan θ→θ and secθ→1.

$\begin{pmatrix}I_{ideal} \\Q_{ideal}\end{pmatrix} = {\begin{pmatrix}1 & 0 \\\theta & 1\end{pmatrix}\begin{pmatrix}I \\Q\end{pmatrix}}$

This means that the phase mismatches between in-phase components andquadrature components are compensated by performing lineartransformation on the in-phase components and quadrature components. Therelation is realized as a form of circuit by the phase compensator 181connected between the output terminal of the orthogonality detector 101and the output terminal of the quadrature demodulation circuit sectionas shown in FIG. 4. As described above, phase mismatch compensation canbe achieved by a feedback loop in a structure shown in FIG. 4.

The phase compensator 181 comprises the multiplier 122 of which inputterminal is connected to any of the output terminal of the orthogonalitydetector 101 and an in-phase component output terminal (output terminalof the low pass filter 132) and quadrature component output terminal(output terminal of the low pass filter 133) of the quadraturedemodulation circuit section, and the adder 191 of which input terminalis connected to the output terminal of the multiplier 122 and the otherone of the in-phase component output terminal and quadrature componentoutput terminal of the quadrature demodulation circuit section. FIG. 4shows an example of structure of which the in-phase component outputterminal (output terminal of the low pass filter 132) of the quadraturedemodulation circuit section is connected to the input terminal of themultiplier 122, and the quadrature component output terminal (outputterminal of the low pass filter 133) is connected to the input terminalof the adder 191. This embodiment is not limited only to this structure.It also includes, for example a structure of which the quadraturecomponent output terminal and in-phase component output terminal of thequadrature demodulation circuit section are exchanged each other, thatis, the quadrature component output terminal (output terminal of the lowpass filter 133) is connected to the input terminal of the multiplier122, and the in-phase component output terminal (output terminal of thelow pass filter 132) is connected to the input terminal of the adder191.

According to the present embodiment, orthogonality can be detectedquickly compared to a conventional case based on the cross correlationby using the outputs of the low pass filter 131 as index of theorthogonality, and accordingly, as similarly as in the second and thirdembodiments, a quadrature demodulator capable of quickly outputtingsignals consisting of the in-phase and quadrature components compensatedin the phase mismatches can be realized. The quadrature demodulatorhaving the above mentioned effect can be realized without installationof a variable phase shifter.

Fifth Embodiment

FIG. 5, illustrating a fifth embodiment according to the presentinvention, shows a block diagram of a quadrature demodulator in samplingquadrature demodulation manner (sampling quadrature demodulator)comprising a circuit for compensating group delay difference betweenin-phase and quadrature signals. Components 151, 171, 201, 202, 211constitute a sampling quadrature demodulation circuit section assimilarly as the components shown in FIG. 12. Group delay difference of,for example ¼ sample is compensated for a group delay differencecompensator consisting of a fixed delay device 221 and a digital filter291 for fixed delay to differentiate the delay between the in-phasesignals and quadrature signals.

The sampling quadrature demodulator of the present embodiment comprisesa sampling quadrature demodulation circuit section sampling the inputsignals at the frequency of 1/n of the carrier frequency of the inputsignals, synchronizing the timing between the in-phase and quadraturecomponents of the sampled input signals, and outputting generallysynchronized in-phase digital signals and quadrature digital signals,and the group delay difference compensator being connected to the outputterminal of the sampling quadrature demodulation circuit section,differentiating the delay difference between the in-phase digitalsignals and quadrature digital signals, and compensating the group delaydifference between demodulated in-phase components and quadraturecomponents.

The sampling quadrature demodulation circuit section comprises a localoscillator 151, a fixed phase shifter 171 connected to the outputterminal of the local oscillator 151, a first A/D converter 201connected to the output terminal of the local oscillator 151 commonlywith the fixed phase shifter 171 and to the input terminal of thesampling quadrature demodulation circuit section for outputting in-phasedigital signals by sampling the input signals at 1/n of the carrierfrequency of the input signals of the sampling quadrature demodulationcircuit section, a second A/D converter 202 connected to the outputterminal of the fixed phase shifter 171 and to the input terminal of thesampling quadrature demodulation circuit section for outputtingquadrature digital signals by sampling the input signals at 1/n of thecarrier frequency of the input signals, and a latch 211 connected to theoutput terminal of the local oscillator 151, the output terminal of thefirst A/D converter 201 and the output terminal of the second A/Dconverter 202 for synchronizing the timing between the in-phase digitalsignals supplied from the first A/D converter 201 and the quadraturedigital signals supplied from the second A/D converter 202.

The group delay difference compensator comprises a fixed delay device221 connected to the in-phase digital signal output terminal of thelatch 211 and a digital filter 291 for fixed delay connected to thequadrature digital signal output terminal of the latch 211. The delayquantity provided to the in-phase signals and quadrature signals can beof any combinations as long as the combination having the difference of,for example ¼ sampling time. However, a digital filter having smalldelay time is hardly used for group delay compensation since such afilter has ripples for the delay property or amplitude property.Accordingly, as shown in FIG. 5, it would be practical that the delaytime of the fixed delay device or digital filter for fixed delay is setlarger than one sampling time, and the difference of delay quantityprovided to the in-phase signals and quadrature signals is set, forexample ¼ sampling time.

The input signals of the sampling quadrature device are converted intoin-phase digital signals and quadrature digital signals at a samplingquadrature demodulation section consisting of the local oscillator 151,the fixed phase shifter 171, and the A/D converters 201, 202. Since theoutputs of the first and second A/D converters 201, 202 are shifted by,for example ¼ sample in sampling timing, the timing is synchronized bythe latch 211 for enabling afterward digital processing. Then theoutputs of the latch 211 are processed by the digital signal processor.The group delay difference of predetermined times (for example, ¼sampling time) between the in-phase and quadrature components of thedemodulated signals is compensated by differentiating the delays of thein-phase and quadrature signals at the group delay differencecompensator into which the outputs of the latch 211 are input. Using theabove mentioned structure, the group delay difference between thein-phase and quadrature signals can be compensated for the samplingquadrature demodulator.

According to this embodiment, the group delay difference between thein-phase and quadrature signals can be compensated in the samplingquadrature demodulation manner. For the effect of the group delaydifference in the sampling quadrature demodulation manner, EVM (ErrorVector magnitude) would be 2.6% when GMSK modulated waves are receivedby an A/D converter operated at the frequency of 8 times of a symbolrate while in this embodiment, the EVM can be compensated.

Sixth Embodiment

FIG. 6, illustrating a sixth embodiment according to the presentinvention, shows a block diagram of a sampling quadrature demodulatorhaving a circuit for compensating group delay difference and orthogonalerrors between in-phase and quadrature signals generally coincidently.Components 151, 171, 201, 202, 211 221, 291 constitute a quadraturedemodulator (sampling quadrature demodulator) in the sampling quadraturedemodulation manner having group delay difference compensation functionsas similarly as the components shown in FIG. 5. Components 101, 111,112, 121, 131 122, 181, 191 constitute a compensator of phase mismatchescomprising an orthogonality detector and a phase compensator assimilarly as in the fourth embodiment.

The sampling quadrature demodulator of this embodiment comprises, inaddition to the structure of the fifth embodiment, an orthogonalitydetector 101 detecting the orthogonality between in-phase digitalsignals as outputs of the fixed delay device 221 and quadrature digitalsignals as outputs of the digital filter 291 for fixed delay; and aphase compensator 181 connected between the output terminal of theorthogonality detector 101 and the output terminals of fixed delaydevice 221 and the digital filter 291 for fixed delay, and performinglinear transformation on the in-phase digital signals and the quadraturedigital signals. The phase mismatches between in-phase and quadraturecomponents of the outputs of the sampling quadrature demodulator iscompensated by performing linear transformation on the in-phase digitalsignals and the quadrature digital signals by the phase compensator 181.

The phase compensator 181 comprises the multiplier 122 of which inputterminal is connected to the output terminal of the orthogonalitydetector 101 and to the output terminal of the fixed delay device 221,and the adder 191 of which input terminal is connected to the outputterminal of the multiplier 122 and to the output terminal of the digitalfilter 291 for fixed delay. According to this embodiment, the groupdelay difference and the phase mismatches can be compensated generallycoincidently as shown in FIG. 6. Since the entire compensation isdigitally processed, it can be aggregated inside one DPS (Digital SignalProcessor). In addition, differently from analog processing, it would beeasy to independently set respective circuit constants.

Seventh Embodiment

FIG. 7, illustrating a seventh embodiment according to the presentinvention, shows a block diagram of a sampling quadrature demodulatorhaving a circuit for compensating group delay difference and orthogonalerrors between in-phase and quadrature signals generally coincidently inthe sampling quadrature demodulation manner. In this embodiment, thegroup delay difference depending on the phase mismatches would be alsocompensated. The only different point from FIG. 6 is that the digitalfilter 291 for fixed delay as shown in FIG. 6 is replaced to a variabledelay device 231 as shown in FIG. 7 and the delay quantity is controlledaccording to the outputs of the orthogonality detector 101.

The variable delay device can be structured as described below. First,the delay is provided by the digital filter, and the correspondencebetween the phase mismatches and the tap coefficients of the digitalfilter are previously designed. Such operations are performed over therange of the phase mismatch compensation, and a correspondence table isprepared and stored in a memory. In actual operation, desired groupdelay property can be obtained by retrieving the correspondence tablefrom the input phase mismatches, reading out the tap coefficients, andsetting the tap coefficients to the digital filter.

Eighth Embodiment

FIG. 8, illustrating an eighth embodiment according to the presentinvention, shows a block diagram of a sampling quadrature demodulatorhaving a circuit for compensating group delay difference, phasemismatches between in-phase and quadrature signals, and conversion gainerrors generally coincidently in the sampling quadrature demodulationmanner. In FIG. 8, a multiplier 123, a low pass filter 132, an adder 192and amplitude square computing units 241, 242 are added to thecomponents as shown in FIG. 7.

The difference of conversion gain between in-phase and quadraturesignals is detected by calculating the difference of outputs of theamplitude square computing units 241, 242 at the adder 123. Sinceoutputs of the adder 123 include variation due to signal modulation, thevariation is removed by the low pass filter 132 and the quadraturecomponents are amplified based on the result. Thus, compensation isperformed so as to make identical the conversion gain of the in-phasesignals and that of the quadrature signals.

For checking convergence of compensation of the phase mismatches, asimulation was performed using GMSK modulated waves with carrierfrequency of eight times of operation frequencies of the A/D converters201, 202. For comparison with a conventional orthogonality detector, asimulation was also performed in the block diagram in FIG. 10. FIG. 10is the same as FIG. 8 except for absence of wide band pass filters 111,112. That is, in FIG. 10, a conventional manner based on crosscorrelation is adopted for detecting orthogonality. The result ofproviding the angle of 10 degrees as an initial value for the phaseerrors is shown in FIG. 14. In the conventional manner based on crosscorrelation, RMS (Root-Mean-Square) phase error after convergence is 3.4degrees while in the manner according to the present invention based oncovariance, RMS phase error after convergence is as low as 0.3 degree,and thus the effect of the present invention can be confirmed.

In FIG. 15, the relation between RMS phase errors by cutoff frequenciesof high pass filters 111, 112 is provided in the same condition. Thehorizontal axis is cutoff frequency normalized with the modulationfrequency, and the longitudinal axis is RMS phase errors afterconvergence. The loop gain of a feedback loop is changed, and thedifference of time constant relating to the convergence was alsoconsidered. A linear IIR (Infinite Impulse Response) filter was used forthe high pass filter. For reference, RMS phase errors compensated in theconventional manner are also shown.

For GMSK modulated waves, it has been determined that the cutofffrequency of the high pas filter should be optimally set to be 0.5-0.6time of the modulated waves. The result might be different depending onthe used modulation manner. In a receiver receiving signals with manykinds of modification manner, the cutoff frequency of the high passfilter should be changed according to the modulation manner for thereceived signals. As shown in FIG. 8, constituting the orthogonalitydetector according to the present invention enables a flexible change inthe cutoff frequency. FIG. 16 shows a diagram illustrating theconvergence of error compensation of conversion gain. As initial valuefor conversion gain difference, an amplitude ratio between in-phase andquadrature signals was provided. It can be confirmed that the errorsafter convergence is restrained within +/−1%.

1. A quadrature demodulator, comprising: a quadrature demodulationcircuit section; and an orthogonality detector into which outputs of thequadrature demodulation circuit section are input, wherein thequadrature demodulation circuit further comprises a circuit forcompensating phase mismatch between in-phase components and quadraturecomponents from outputs of the quadrature demodulation circuit sectionbased on outputs of the orthogonality detector, wherein theorthogonality detector further comprises a first high pass filter forremoving low frequency components of in-phase components of the signalinput I from the quadrature demodulation circuit section, a second highpass filter for removing low frequency components of quadraturecomponents of the signal input Q from the quadrature demodulationcircuit section, a multiplier for multiplying outputs of the first andsecond high pass filters with each other, and a low pass filter forremoving high frequency components from an output of the multiplier,wherein the orthogonality detector is configured to detect anorthogonality error between the in-phase and quadrature components ofthe signals input from the quadrature demodulation circuit section basedon the output of the low pass filter, and to compensate the output ofthe quadrature demodulation circuit section based on the orthogonalityerror, to provide a compensated output of the quadrature demodulator,wherein an output from the first high pass filter is (I−E[I]), an outputfrom the second high pass filter is (Q−E[Q]), E being an average, andwherein the output from the multiplier comprises a covariance E[(I−E[I])(Q−E[Q])], wherein the quadrature demodulation circuit section comprisesa local oscillator, a first mixer outputting in-phase components of theinput signal of the quadrature demodulation circuit section, a secondmixer outputting quadrature components of the input signal of thequadrature demodulation circuit section, and a variable phase shifterconnected between an output terminal of the orthogonality detector andan input terminal of one of the first and second mixers, wherein thevariable phase shifter changes a phase shift quantity based on theoutput of the orthogonality detector, and controls an input phase of themixer to which the output terminal of the variable phase shifter isconnected.
 2. The quadrature demodulator according to claim 1, whereinan input terminal of the variable phase shifter is connected to theoutput terminal of the local oscillator commonly with one of the firstand second mixers, an output of the local oscillator is connected to theother of the first and second mixers, and the output of the localoscillator of which phase is controlled by the variable phase shifter issupplied to the one of the first and second mixers to which the outputterminal of the variable phase shifter is connected.
 3. The quadraturedemodulator according to claim 1, wherein an input terminal of thevariable phase shifter is connected to the output terminal of thequadrature demodulation circuit section, the output of the localoscillator is connected to the input terminal of one of the first andsecond mixers, and the input of the quadrature demodulation circuitsection of which phase is controlled by the variable phase shifter issupplied to the one of the first and second mixers to which the outputterminal of the variable phase shifter is connected.
 4. A quadraturedemodulator, comprising: a quadrature demodulation circuit section; andan orthogonality detector into which outputs of the quadraturedemodulation circuit section are input, wherein the quadraturedemodulation circuit further comprises a circuit for compensating phasemismatch between in-phase components and quadrature components fromoutputs of the quadrature demodulation circuit section based on outputsof the orthogonality detector, wherein the orthogonality detectorfurther comprises a first high pass filter for removing low frequencycomponents of in-phase components of the signal input I from thequadrature demodulation circuit section, a second high pass filter forremoving low frequency components of quadrature components of the signalinput Q from the quadrature demodulation circuit section, a multiplierfor multiplying outputs of the first and second high pass filters witheach other, and a low pass filter for removing high frequency componentsfrom an output of the multiplier, wherein the orthogonality detector isconfigured to detect an orthogonality error between the in-phase andquadrature components of the signals input from the quadraturedemodulation circuit section based on the output of the low pass filter,and to compensate the output of the quadrature demodulation circuitsection based on the orthogonality error, to provide a compensatedoutput of the quadrature demodulator, wherein an output from the firsthigh pass filter is (I−E[I]), an output from the second high pass filteris (Q−E[Q]), E being an average, and wherein the output from themultiplier comprises a covariance E[(I−E[I]) (Q−E[Q])], wherein theorthogonality error between in-phase components and quadraturecomponents of the output of the quadrature demodulation circuit sectionis compensated by performing linear transformation on the in-phasecomponents and quadrature components of the signal input from thequadrature demodulation circuit section.
 5. The quadrature demodulatoraccording to claim 4, wherein the linear transformation is performed bya phase compensator connected between an output terminal of theorthogonality detector and output terminals of the quadraturedemodulation circuit section.
 6. The quadrature demodulator according toclaim 5, wherein the phase compensator comprises: a multiplier having aninput terminal connected to the output terminal of the orthogonalitydetector and to one of an in-phase component output terminal and aquadrature component output terminal of the quadrature demodulationcircuit section; and an adder having an input terminal connected to anoutput terminal of the multiplier and to the other one of the in-phasecomponent output terminal and the quadrature component output terminalof the quadrature demodulation circuit section.